Actually, I threw a "Terrible Twos" theme for my daughter's birthday a couple of years ago, but I think it would work for a 5 year old boy. The idea of the party is to get a messy as possible without getting into trouble, which I think 5 year old boys can manage without a problem. We had craft supplies so that kids or parents could make their own t-shirts and picture frames. We had a make your own sundae bar. For the games, there was "digging for worms" where we laid out a plastic wrap on the floor (I guess you could do it on a table, too), spread a layer of chocolate pudding, a layer gummy worms, and a layer or two of cookie crumbs. For a given amount of time, contestants (young and old--my 60-something year old great aunt was down there) with their hands behind their back, pick out the worms using their teeth. The person who gets the most worms wins a prize. Another game involved pairing people. One dons a shower cap with shaving cream on top and kneels or stands approx. 5 ft. from his partner. The partner then tries to get as many Cheetos on his partners cap as possible in a given amount of time, and the pair who has the most, wins. There are a ton of those messy games on the internet. I put on the invitation that people should not wear their best clothes, because they were sure to get messy. My daughter's birthday is in winter, so I used my father's empty basement, as there was little in the way of obstacles, and easier clean-up; perhaps you will find that this idea works outside as well.